Seike K et al. (2017), Using tsunami deposits to determine the maximum...

ECB-ART-45723

PLoS One 2017 Aug 04;128:e0182753. doi: 10.1371/journal.pone.0182753.

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Using tsunami deposits to determine the maximum depth of benthic burrowing.

Seike K , Shirai K , Murakami-Sugihara N .

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The maximum depth of sediment biomixing is directly related to the vertical extent of post-depositional environmental alteration in the sediment; consequently, it is important to determine the maximum burrowing depth. This study examined the maximum depth of bioturbation in a natural marine environment in Funakoshi Bay, northeastern Japan, using observations of bioturbation structures developed in an event layer (tsunami deposits of the 2011 Tohoku-Oki earthquake) and measurements of the radioactive cesium concentrations in this layer. The observations revealed that the depth of bioturbation (i.e., the thickness of the biomixing layer) ranged between 11 and 22 cm, and varied among the sampling sites. In contrast, the radioactive cesium concentrations showed that the processing of radioactive cesium in coastal environments may include other pathways in addition to bioturbation. The data also revealed the nature of the bioturbation by the heart urchin Echinocardium cordatum (Echinoidea: Loveniidae), which is one of the important ecosystem engineers in seafloor environments. The maximum burrowing depth of E. cordatum in Funakoshi Bay was 22 cm from the seafloor surface.

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	Fig 1. Schematic representation of changes in sedimentary features in an area affected by the 2011 tsunami from before the tsunami to several years after the event.(A) The seafloor situation before the 2011 tsunami. The substrate was bioturbated. (B) Situation during the tsunami. The seafloor was eroded by the strong tsunami current. (C) Just after the 2011 tsunami. The seafloor is covered with tsunami deposits. The radiocesium (134Cs and 137Cs) released from the Fukushima Nuclear Accident was deposited on the seafloor surface. (D) Expected seafloor situation in the years following the 2011 tsunami. Bioturbated layer lacks physical sedimentary structures and carries the radiocesium due to vertical biomixing of the sediments following recolonization by benthic fauna. Layer 1: tsunami deposits, bioturbated after recolonization by benthic fauna after the event. Layer 2: unbioturbated tsunami deposits, showing well-defined physical sedimentary structures such as parallel laminations. Layer 3: pre-tsunami deposits.
	Fig 2. Map of the study area showing the locations of sampling sites.
	Fig 3. Temporal change in test width of Echinocardium cordatum in Funakoshi Bay before and after the 2011 tsunami.The E. cordatum population disappeared in 2011 and began to recolonize from early 2012. The test width had reached the same size as before the tsunami by September 2014, when the sediment cores were collected.
	Fig 4. Grain-size distributions, concentrations of 137Cs, CT images, photographs, and columnar sections for the cores.Layer 1, the upper part of the cores, was bioturbated. Layer 2, the sediments between the base of Layer 1 and the erosional surface (the coarse-grained bed), shows well-defined physical sedimentary structures such as parallel laminations. Layer 3, the sediments beneath the erosional surface, show neither physical sedimentary structures nor obvious bioturbation structures. nd: not detected. m: mud. s: sand. g: gravel.
	Fig 5. Close-up view of the burrows produced by Echinocardium cordatum.(A) CT image of the upper part of core F-2. (B) Sketch of (A). The transverse width of the E. cordatum burrows is ~3 cm.

References [+] :

Abe, Impacts of the 2011 tsunami on the subtidal polychaete assemblage and the following recolonization in Onagawa Bay, northeastern Japan. 2015, Pubmed